Collagen is the principal protein component of the extra-cellular matrix. In mammals, collagen sometimes constitutes as much as 60% of the total body protein. It comprises most of the organic matter of skin, tendons, bones and teeth, and occurs as fibrous inclusions in most other body structures. Collagen is also the principal ingredient in fish skins.
There are many types of collagen which differ from each other to meet the requirements of various tissues. Currently, animal collagen types I through XIX have been discovered, and of these, collagen types I through V are used in a variety of ways as medical and cosmetic materials as well as food supplements. In particular, type I collagen is used most commonly as an extra cellular matrix. These collagens are extracted and purified from the connective tissue of various organs such as skin, bone, cartilage, tendon, and viscus of animals such as cows, pigs, birds, kangaroos and so forth by acidic solubilization, alkaline solubilization, neutral solubilization and or enzymatic solubilization.
In alkaline solubilization, salt solutions are used to extract the salt-soluble fractions of collagen which is a small fraction with minimal crosslinking.
Dilute acid solutions are also been used to extract collagen from young rapidly growing tissues. The acid-soluble fraction is slightly greater than the salt soluble fraction. However, at times, the extraction solutions may be more dangerous to handle due to their acidic pH (Gross et al., Proc. Natl. Acad. Sci U.S.A., 41, pp 1-7 (1955) and Davison et al, Conn. Tiss. Res., 1, pp 205-216 (1972)).
Enzymatic solubilization, e.g., use of pepsin, is also employed to solubilize another fraction of collagen. Enzyme extraction is preferable in many instances because this methodology produces increased yields and higher purity collagen. However, enzyme extraction suffers the disadvantage of producing partially degraded collagen, i.e., the extraction enzyme cleave the collagen molecule at the terminal non-helical regions which contain the inter-collagenous cross-linkages. It has been found that collagen extracted by use of the enzyme pepsin, a frequently used enzyme for the production of enzyme-extracted collagen, produces living tissue equivalents which are undesirably weak for certain applications, e.g., those which involve substantial mechanical handling of the tissue equivalent.
Thus, there is the need for improved collagen compositions and methods of preparing such compositions.